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minerals to help ensure our domestic economic security, assessing the quality of the Nation's surface- and ground-water resources, and applying our extensive knowledge of cartography and remote sensing to develop innovative approaches to the solution of many land- and resourcemanagement concerns.
Conducting the research and investigations of our multidisciplinary programs is only half of our mission. Our work is not done until we disseminate the results of our investigations and research to those who need earth-science information to solve land-use problems, to better develop and manage the Nation's natural resources, and to understand and mitigate the effects of natural hazards. Through these efforts to effectively communicate earth-science information, the USGS remains committed to conducting science that serves the Nation.
Few Federal agencies can claim the longevity of service or the stability of mission of the U.S. Geological Survey. For 109
years now we have been the Nation's earth scientists, fulfilling our original mission to conduct systematic and scientific "classification of the public lands, and examination of the geological structure, mineral resources, and products of the national domain." We are aware of the trust which has been placed in us by the taxpayers and legislators of this country to fulfill that mission.
With that primary mission and that trust in mind, the USGS dedicated itself during the year to the activities documented in the following pages. As much as this volume is an opportunity to look back, it is also a challenge to look to the future and to recommit ourselves and our science to providing to the Nation the information by which the public and its officials make informed decisions concerning the wise use of our Nation's resources. It is with pride of accomplishment and dedication of purpose that I present to you this Yearbook of the U.S. Geological Survey.
International Cooperative Program Studies the Earth's Crust Beneath the Great Lakes
By William F. Cannon
The Great Lakes cover about 98,000 square miles of the Earth's surface, an area approximately equal to the combined land areas of the States of Wisconsin and Michigan. Because the waters of the lakes conceal the bedrock beneath them from direct, conventional geologic mapping and study, the lakes represent a large gap in our understanding of the geologic nature and evolution of the central part of the North American Continent. Many major geologic features, well studied on land, project beneath the lakes, and many key areas where these major features meet or intersect are beneath the lakes. Geologists and geophysicists have realized for many years that the geologic relationships hidden beneath the lake waters hold important keys to unraveling the geologic evolution of both the immediate region and the North American Continent in general.
In addition to the scientific questions, many practical problems can be better addressed by defining the geology beneath the lakes. Geological data can be used to identify the distribution and magnitude of energy and mineral resources, the location and severity of geologic hazards, and the suitability of areas for specialized development, such as nuclear plants and toxicwaste isolation sites. All of these issues are major societal concerns, especially in view of the many large and expanding population centers along the lake shores in both the United States and Canada. The geology beneath the lakes, immediately adjacent to these population centers, is probably the least well known of any place in the United States.
Because of these scientific and practical concerns, a consortium of U.S. and Canadian scientists was organized in November 1986 to renew efforts to study the subsurface beneath the Great Lakes. The consortium, called the Great Lakes International Multidisciplinary Program on Crustal Evolution-GLIMPCE, for short—was organized by the U.S. Geological Survey (USGS) and the Geological Survey of Canada (GSC). It includes a wide variety of earth scientists from the two organizing agencies, from State and Provincial geological surveys, and from many U.S. and Canadian universities. The members represent an enormous source of information and expertise on the geology surrounding the lakes, as well as the geology as it is inferred beneath the lakes as a result of geophysical surveys. The members have agreed to share existing expertise and, more significantly, to share capabilities, equipment, and costs for conducting critically needed new surveys.
In August and September of 1986, the first major surveys of GLIMPCE were conducted. They consisted of "marine" seismic surveys in Lake Superior, Lake Huron, and Lake Michigan, and aeromagnetic surveys of Lake Huron. Aeromagnetic surveys of Lake Superior were conducted in the summer of 1987.
The USGS and GSC jointly contracted the services of a commercial seismic surveying vessel that had been performing surveys off the Atlantic coast and brought it into the lakes. The ship acquired 850 miles of seismic reflection profiles (fig. 1). In this technique, air guns that release blasts of compressed air beneath the lake surface generate strong sound waves, which propagate to the lake bottom and are transmitted deep into and through the Earth's crust. Portions of these waves are reflected from contacts of rock bodies that have contrasting acoustic properties, and these "echoes" are recorded on a series of hydrophones towed behind the ship (fig. 2).
Sophisticated computer processing of these recorded echoes at USGS facilities in Denver, Colorado, and at GSC facilities in Ottawa, Ontario, reconstructed a picture of the distribution of rock bodies at depths as great as 36 miles. From this picture, scientists were able to construct the first cross sections of the geology beneath the lakes.
At the same time the reflection profiles were being acquired, teams of geophysicists were recording the air gun signals on the land around the lakes, on islands, and with special seismometers deployed on the lake bottom. This phase of the experiment was conducted by per